>I believe the kubotan is illegal in some countries - UK in particular.< What an absolutely stupid law, wow I am glad I do not live in the UK. Why don't they go ahead and ban pens, wine bottle openers, pop bottles and candy bars too. Far too dangerous. Idiocy, where did the uk go wrong.. I digress.
Umbrellas are good for poking. Don't try grappling or striking with your average everyday umbrella, it's too lightweight and will break easily. But stabbing motions work great especially to vital areas of the body, it'll withstand a few of those.
Metal pens are a good substitute for kubotan, and good for hitting weak points or throwing as a distraction into another move. Don't use piano wire on someone unless you plan on risking killing them, that stuff cuts through skin way easily and you could find yourself slicing their arteries or something accidentally..
Keys are good for slashing someones face/eyes/neck with.
I've used stuff from a bag of groceries before. Cans of soup pack a lot of kinetic energy, I knocked a guy out by throwing one at his head when he encroached on my comfort zone a few years ago after a night time shopping spree. Of course any hard objects like bottles, etc... would work fine as well.
CD's broken in half can be used to scratch the hell out of someones face but it's a pretty desperate measure. But the edges on the corners are almost as sharp as broken glass. Or you can break it into 3 pieces and use one as a crude shank.
Sharpened or broken pencils can be used to stab the neck / side of the head or eyes. Kind of useless unless you know the right spots to hit as generally a pencil will break before puncturing skin (or skin like material which is what I tested on).